重新检查高绝对值SDSS质量金属丰度离群值：高N/O、演变的Wolf-Rayet Galvanic？
Re-examining High Abundance SDSS Mass-Metallicity Outliers: High N/O, Evolved Wolf-Rayet Galaxies?
摘要：
我们提出了四个矮星系的新MMT光谱观测，这些矮星系代表了斯隆数字巡天（SDSS）观测到的较大样本，并被Peeples等人（2008）确定为质量-金属丰度关系中的低质量，高氧丰度异常值。Peeples等人（2008年）表明，这四个天体（金属丰度估计值为8.5 =< 12 + log（O/H）=< 8.8）的氧丰度与M_B光度-金属丰度关系的偏差为0.4-0.6 dex。我们的新观测扩展了波长覆盖范围，包括[OII] 3726，3729双峰，这增加了氧丰度估计的杠杆作用，并允许测量N/O比。所有四个光谱都是低激发的，具有相对高的N/O比（N/O >~ 0.10），其中每一个都倾向于使基于强发射线的估计偏向于高氧丰度。这些光谱都落在一个政权的“标准”强线的金属丰度测定方法没有很好地校准经验或光电离建模。通过将我们的光谱直接与光电离模型进行比较，我们估计氧丰度在7.9 =< 12 + log（O/H）=< 8.4的范围内，与质量-金属丰度关系的分散一致。我们讨论了这些星系的物理性质，导致他们的不寻常的光谱（和以前的分类为离群值），发现他们的低激发，升高的N/O，和强大的巴耳末吸收是一致的，从过去的“沃尔夫-拉叶星系”阶段的星系演化的预期属性。我们将我们的结果与Peeples等人（2008）的“主要”样本进行比较，并得出结论，它们是异常值，主要是由于氮相对于氧的富集，而不是由于其质量或光度的氧丰度异常高。
英文摘要：
We present new MMT spectroscopic observations of four dwarf galaxies representative of a larger sample observed by the Sloan Digital Sky Survey (SDSS) and identified by Peeples et al. (2008) as low-mass, high oxygen abundance outliers from the mass-metallicity relation. Peeples et al. (2008) showed that these four objects (with metallicity estimates of 8.5 =< 12 + log(O/H) =< 8.8) have oxygen abundance offsets of 0.4-0.6 dex from the M_B luminosity-metallicity relation. Our new observations extend the wavelength coverage to include the [OII] 3726,3729 doublet, which adds leverage in oxygen abundance estimates and allows measurements of N/O ratios. All four spectra are low excitation, with relatively high N/O ratios (N/O >~ 0.10), each of which tend to bias estimates based on strong emission lines toward high oxygen abundances. These spectra all fall in a regime where the "standard" strong line methods for metallicity determinations are not well calibrated either empirically or by photoionization modeling. By comparing our spectra directly to photoionization models, we estimate oxygen abundances in the range of 7.9 =< 12 + log(O/H) =< 8.4, consistent with the scatter of the mass-metallicity relation. We discuss the physical nature of these galaxies that leads to their unusual spectra (and previous classification as outliers), finding their low excitation, elevated N/O, and strong Balmer absorption are consistent with the properties expected from galaxies evolving past the "Wolf-Rayet galaxy" phase. We compare our results to the "main" sample of Peeples et al. (2008) and conclude that they are outliers primarily due to enrichment of nitrogen relative to oxygen, and not due to unusually high oxygen abundances for their masses or luminosities.
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